Method and apparatus for precisely measuring accelerating voltages applied to x-ray sources

ABSTRACT

A method of measuring an accelerating voltage applied to an x-ray source to produce x-rays includes applying an accelerating voltage to an x-ray source to produce x-ray radiation having an axis; diffracting first and second portions of the x-ray radiation that are symmetrically disposed relative to an axis with a single crystal material to produce two spectra of the x-ray radiation, each spectrum including continuous x-ray radiation having an end point energy at the maximum energy of the x-ray radiation; forming an image of the spectra of the x-ray radiation including respective end point energies; and measuring the separation of the respective end point energies of the spectra of the image and, thereby, determining the accelerating voltage applied to the x-ray source.

FIELD OF THE INVENTION

The present invention concerns a method of accurately measuring theaccelerating voltage applied to an x-ray tube to accelerate electronsagainst a target, resulting in the production of x-ray radiation. Theinvention also relates to an apparatus for rapidly and preciselymeasuring the accelerating voltage.

BACKGROUND OF THE INVENTION

In medical applications, x-ray radiation is conventionally producedwithin an evacuated tube. Electrons are generated by a source, such as aheated filament, and accelerated by a high voltage applied to anaccelerating electrode. The accelerated electrons are directed towardand impact on a metal target, typically tungsten or molybdenum,resulting in the production of x-ray radiation. Typically, the peakaccelerating voltage (kVp) is at least 15,000 volts and can be up to tentimes higher. The accelerating voltage employed depends upon thediagnostic or therapeutic use being made of the x-ray radiation. Thex-ray radiation increases in penetrating power with an increase in theaccelerating voltage applied to the x-ray source.

In diagnostic applications of x-rays, the amount of radiation thatpasses through an object being analyzed with x-rays determines thecontrast of a radiographic image on a fluoroscopic screen orphotographic film. In some applications, moderate variations in x-raypenetrating power due to variations in the accelerating voltage appliedto the x-ray source do not significantly affect x-ray analysis. Forexample, small variations in the contrast of photographic x-ray imagesof broken bones do not affect the ability of a radiologist and/ororthopedist to determine the location and nature of a bone fracture.However, in other applications of x-ray imaging, the contrast of aphotographic x-ray image can be critical to accurate diagnosis of adisease or disorder. Two important examples where radiographic contrastcontrol is of substantial or critical importance are mammography andcoronary angiography. In those applications, variations in imagecontrast caused by even moderate variations in accelerating voltage canresult in erroneous diagnoses. Therefore, precise measurement of theaccelerating voltage applied to an x-ray tube is essential to properdiagnosis and treatment of important, potentially life-threateningdiseases and disorders.

Measurement of the relatively high voltages applied to x-ray tubes usedas x-ray radiation sources in radiological equipment presents numeroustechnological problems. Direct measurement of such high voltages is, inprinciple, straightforward for constant potentials and low frequencyaccelerating voltages. In the early development of x-ray equipment,power supplies for producing such high voltages were relatively largeand frequently encased in a liquid dielectric medium, such as oil.Nevertheless, the output voltage terminals of the accelerating voltagepower supply were accessible. In order to measure the acceleratingvoltage applied to an x-ray tube, a high tension voltage divider wasconnected across the output terminals of the accelerating voltage powersupply so that a portion of the high voltage could be measured. Theaccelerating voltage could be calculated based upon the voltage dividingratio of the voltage divider. These voltage dividers function well forthe direct current and the low frequency components of the acceleratingvoltage kVp produced by older x-ray equipment. However, modern x-rayequipment produces accelerating voltages with high frequency componentsand complex waveforms that conventional voltage dividers cannotaccurately measure without specific calibration of the dividers.

Typically, medical x-ray equipment includes a voltage control forvarying the accelerating voltage so that the equipment can be used invarious applications. The equipment usually includes an indicator, suchas a dial or a meter, for indicating the accelerating voltage. Theindicator is not directly connected to the accelerating voltage producedby the high voltage power supply but is actually connected to measure alower voltage input to the accelerating voltage power supply.Historically, the accelerating voltage indicator was calibrated byconnecting a voltage divider directly to the accelerating voltage powersupply, setting the voltage control to a number of positions, measuringthe actual accelerating voltage across the voltage divider for eachposition of the voltage control, and adjusting the indicator asnecessary. The voltage divider is not present during medical use of theequipment so that a radiological technician depends upon the accuracy ofthe calibration of the indicator when setting the accelerating voltagefor x-ray imaging or treatment. With the passage of time, the indicatorcan become inaccurate and periodic recalibrations are required torestore the accuracy of the indicator. In fact, governmental regulatorybodies frequently require periodic recalibration of x-ray equipment tomaintain minimum standards of health safety.

Although power supplies in older x-ray equipment typically directlyproduced the pulsating, i.e., with some degree of ripple, direct currentaccelerating voltages using transformers, rectifiers, and capacitivefilters, modern x-ray accelerating voltage power supplies are morecomplex. Modern x-ray equipment power supplies may employ toroidaltransformers and inverters as well as rectifiers operating at variousfrequencies and with pulsed signals having complex waveforms to producethe accelerating voltage. Many parts of the modern power supplies areencapsulated in resins and do not permit access for the connection of ahigh tension voltage divider for calibrating the accelerating voltageindicator of the equipment.

Non-invasive, i.e., without direct electrical connection, techniques areand may have to be employed to calibrate the accelerating voltageindicators of modern x-ray equipment. One known technique employs two ormore metal foils of different thicknesses, generally arranged seriallywith respective x-ray radiation detectors. In most cases the foils aremade of the same materials. The different thicknesses of the foils causedifferent modifications of incident x-ray radiation that reaches thedetectors. The relative intensities of the radiation that penetrates therespective foils is measured and compared, sometimes in a complexmathematical relationship, to determine an empirical correlation betweendifferences in the radiation intensities measured by the respectivedetectors and accelerating voltage. Convenient methods of evaluating themeasured radiation intensities produce results that depend upon thewaveform of the accelerating voltage and, therefore, are not universallyapplicable to all x-ray equipment. Depending upon the voltage applied tothe x-ray tube, the accuracy of voltage measurements using single foilsranges from five to ten percent, for example, ±2 kV when kVp is 20 kV.

An improvement in measurement accuracy as compared to the use of foilsof the same material, at least for measuring a single acceleratingvoltage, can be achieved when filters of different materials are used.Any of the elements having atomic numbers between 40 and 60 may be usedas one of the filters. A second filter having a much lower atomic numberand an x-ray absorption characteristic matched to that of the firstfilter over part of a radiation energy range is used. The singleacceleration voltage calibration point is unique to the specificelements used in the filters. In one commercially available filter pair,cadmium and aluminum are used in tandem with a photodiode detector. Thephotodiode is sensitive to x-rays that penetrate the filters. As thevoltage applied to the x-ray tube is increased, significantly differentx-ray penetration of the cadmium and aluminum filters occurs, asindicated by the photodiode detector, once the peak acceleratingvoltage, kVp, exceeds 26.5 kV. Thus, the cadmium and aluminum filterpair provides a precise indication of a single accelerating voltage.Other pairs of filter materials can be used to identify additionalindividual accelerating voltages precisely although the use of many suchfilter pairs to detect many accelerating voltages is cumbersome.

Improved precision in accelerating voltage measurement can be made usingan ionization spectrometer that measures the shape of the x-ray spectrumnear the end point energy. The spectrum of electromagnetic energyproduced by an x-ray tube has two recognized components. First, thex-ray spectrum includes sharp "lines" of relatively intense x-rayradiation at wavelengths that are characteristic of and well establishedfor various x-ray tube target materials. These radiation components,typically generally identified as Kα and Kβ lines, result from thetransfer of energy from accelerated electrons to atoms of the target,followed by energy transitions between inner shell electron energystates of target atoms that produce x-ray radiation. The energytransitions between well defined energy levels account for the specificenergies of the line components of the radiated x-ray spectrum.

Second, in addition to the sharp lines in the x-ray spectrum, a morebroadly distributed component of continuous x-ray radiation is alsoproduced. This radiation, the so-called Bremsstrahlung, results from thescattering of accelerated electrons by the target accompanied byemission of x-ray radiation having energies equal to the energies givenup by the electrons in the scattering process. The energies given up arenot confined to discrete energies so that a broad x-ray energydistribution, i.e., a continuous x-ray spectrum, is produced. Themaximum energy loss that can occur in the electron scattering processoccurs when all of the kinetic energy of an accelerated electron is lostand is converted to x-ray radiation. Since the kinetic energy of anaccelerated electron equals the electronic charge, e, of the electronmultiplied by the accelerating voltage, that total kinetic energy lossproduces the highest energy x-rays within the continuous component ofthe x-ray spectrum. That energy, which may be measured in terms of themaximum frequency or the minimum wavelength of the x-ray radiation, isreferred to as the end point energy because it is the upper energy limitof, i.e., end point of, the continuous component of the x-ray radiation.

In an ionization spectrometer, the shape of the x-ray spectrum near theend point energy is measured by analyzing charge pulses produced in acrystalline material, such as intrinsic germanium, in response toincident x-ray radiation. However, despite the improved voltagemeasurement accuracy achieved with an ionization spectrometer ascompared to the use of foils and filters, ionization spectrometermeasurements can only be made while a relatively low electron beamcurrent flows in the x-ray tube. Higher currents increase the quantityof x-ray radiation produced and cause overlapping charge pulses in thecrystal that cannot be reliably analyzed. Since the spectrometer canonly analyze one charge pulse at a time, accelerating voltagemeasurement using an ionization spectrometer takes a relatively longtime. The measured results must be extrapolated for practical electronbeam currents, and errors may be introduced in the extrapolation if theaccelerating voltage waveform is current-dependent.

Accordingly, it would be desirable to provide a method and apparatus foraccurately measuring the accelerating voltage applied to an x-ray tube.Most preferably, the apparatus is portable and the method is simple sothat calibration of existing clinical x-ray equipment can be carried outat the site of the equipment, without modification of the equipment, bya technician rather than a scientist, and without undue interruption inthe medical use of the equipment.

SUMMARY OF THE INVENTION

The invention provides a method of precisely measuring the acceleratingvoltage applied to an x-ray tube using a simple apparatus with a directreading taken from a spectrographic image of the radiation produced bythe x-ray tube. The end point energy at the end of the continuouscomponent of the x-ray radiation spectrum is precisely located as anindication of the voltage applied to the x-ray tube. The acceleratingvoltage indicator of an x-ray apparatus is calibrated by measuring theaccelerating voltage at each of a number of accelerating voltages andvoltage indications and producing a correction table or graphcorrelating the indicated accelerating voltage with the actually appliedaccelerating voltage.

According to one embodiment of the invention, an accelerating voltage isapplied to an x-ray source to produce x-ray radiation. A portion of thex-ray radiation is diffracted with a single crystal material to producea spectrum of the x-ray radiation including a continuous x-ray radiationcomponent having an end point energy at a maximum energy of the x-rayradiation. An image of the spectrally dispersed x-ray radiationincluding the end point energy is formed and one of the wavelength andfrequency of the energy end point and, thereby, the accelerating voltageapplied to the x-ray source is determined. Preferably, two portions ofthe x-ray radiation that are symmetrically disposed relative to an axisare dispersed with the same single crystal material to form symmetrical,mirror image spectra of the x-ray radiation. The mirror image spectrainclude respective end point energies. The spacing between the two endpoint energies in the mirror image, indicates, within an accuracy ofabout 0.1 kV, the accelerating voltage applied to the x-ray tube.

The x-ray spectrum or spectra image may be formed on a photographic filmor on a scintillation screen. In a preferred apparatus for employing thenovel method, a scintillation screen is disposed on a charge coupleddevice (CCD) camera so that an image, in light, of the x-ray spectrum orspectra formed by the scintillation screen is converted into anelectrical image by the CCD camera. The electrical image is supplied toa computer for analysis and automatic determination of the acceleratingvoltage applied to the x-ray source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of an x-ray spectrograph used in amethod according to the invention.

FIG. 2 is an image of x-ray spectra produced by the spectrograph of FIG.1.

FIG. 3 is an enlarged view of a portion of one spectrum of FIG. 2.

FIG. 4 is a block diagram of an embodiment of an apparatus according tothe invention for automatically and accurately measuring acceleratingvoltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a plan view, partially in section, illustrating a spectrographand other elements used in an embodiment of the invention. Theillustrated spectrograph is a version of a Rutherford-Andradespectrograph that was first described about 1914. In the apparatus ofFIG. 1, an x-ray tube 1 has an accelerating voltage applied to it from avariable magnitude accelerating voltage source 2. A spectrograph 3having an entrance is placed adjacent the x-ray tube 1. X-ray radiationgenerated in the x-ray tube 1 enters the spectrograph 3 through theentrance. In FIG. 1, the entrance includes two spaced apart openings 5and 6 for admitting two portions of the x-ray radiation. In thisembodiment, the x-ray radiation is generated by a mammography apparatusthat produces a relatively small x-ray source area. In other embodimentsof the apparatus, particularly with broader x-ray source areas, theentrance of the spectrograph may be a slit with knife edges. Radiationportions 7 and 8, admitted through openings 5 and 6, respectively, formacute angles that are symmetrically disposed with respect to an axis ofsymmetry of the spectrograph 3. A lead shield 9 and baffles 10 and 11absorb incident x-ray radiation but allow portions 7 and 8 to betransmitted.

The two portions 7 and 8 of the x-ray radiation produced by the tube 1are incident on a single crystal material 20. As well known in the art,single crystal materials have a periodic structure that diffracts anddisperses incident x-ray radiation provided the spacing, d, betweenlayers of atoms in the single crystal material has an appropriaterelationship with the wavelength of the incident x-rays. X-rays arediffracted at an angle θ by the single crystal material when therelationship nλ=2d sin θ is satisfied, where λ is the wavelength of thex-ray radiation and n is the order of the reflection, assumed here to beone. Since, in a single crystal material, d is fixed for a particularorientation of the single crystal material, incident x-ray radiationhaving a variable wavelength, i.e., energy, is dispersed over a range ofangles. In the dispersion, from the foregoing equation and the knownrelationship that the energy of the incident x-ray radiation isproportional to its frequency and inversely proportional to itswavelength, the constituents of the x-ray radiation are dispersedthrough angles proportional to the inverse sine of the wavelength of theconstituents.

In the apparatus shown in FIG. 1, it is convenient to use amonocrystalline silicon wafer as the single crystal 20, for example,with a (110) orientation, perpendicular to the axis of symmetry ofportions 7 and 8. It may also be important to select a particularcrystalline orientation of the single crystal material 20 transverse to,as well as parallel to, the x-ray radiation. Other single crystalmaterials besides silicon may be used as the dispersing means providedthat the interatomic spacing d of the chosen crystalline orientation ofthe single crystal material is appropriate for dispersing the x-rayradiation.

The dispersed x-rays, i.e., the x-ray spectrum or spectra, pass througha slit 21 and are incident on a film 22, such as a photographic film,that is sensitive to and forms an image of the incident x-ray radiation.The spacing between features of the x-ray spectrum or spectra in theimage formed on the film 22 depends upon the distance, L, between theslit 21 and the film 22. In using the apparatus shown in FIG. 1, withthe two portions 7 and 8, two x-ray spectra are produced in asymmetrical, mirror image form, as illustrated in FIG. 2. In thisapparatus, the spacing between identical features in each of the spectrais equal to

    2L tan [arc sin(λ/2d)]                              (1).

Turning to FIG. 2, an example of mirror image spectra produced when theapparatus of FIG. 1 is used with a photographic film as the film 22 isillustrated. FIG. 2 itself is not the photographic image of the x-rayspectra but an analysis of such an image prepared using a conventionaldensitometer. In FIG. 2, two prominent x-ray radiation lines areapparent in each spectrum. In addition, the continuous radiation x-raycomponent is present between and on both sides of the x-ray lines. Thatcontinuous component is displayed in the spectra as a function of energythat increases from the outside edges of FIG. 2 toward the center ofFIG. 2. In the central part of FIG. 2, the density of the image on thefilm is small and relatively constant. The constant value indicates theabsence or near absence of incident radiation and is a background value.The points of transition from the varying amplitude continuous componentof the x-ray spectra and the essentially constant background are the endpoint energies, one for each spectrum, referred to above. The relativelyconstant background is due, in part, to the shield 9 and the baffle 10that prevent undiffracted x-ray radiation from reaching the film 22 andincreasing the background value, i.e., "fogging" the film 22. The shield9 and the baffle 10 may not be necessary in some situations.

In FIG. 2, the end point energies in each of the spectral halves can bereadily located and the distance between the two end points can beeasily measured. For a fixed spectrograph geometry, the separationbetween the end points can be measured and directly converted to anaccelerating voltage. Alternatively, either or both of the end pointenergies can be converted to an accelerating voltage using the knowledgethat end point energy equals the electronic charge multiplied by theaccelerating voltage.

With regard to the mirror image spectra of FIG. 2, in Equation 1 everyfactor except λ, the wavelength, which can be easily converted toenergy, is known for a fixed geometry spectrograph and a selected singlecrystal material. Thus, when the separation Δ between the end pointenergies of the two spectra is determined from the image shown in FIG.2, the accelerating voltage can be directly calculated. In other words,through the following equations, the peak accelerating voltage, kVp, asa function of the separation, Δ, of the end point energies on thesymmetrical, mirror image spectra of FIG. 2 can be readily determinedusing fundamental physical relationships.

    Δ=2L tan[arc sin(λ/2d)]

    Δ=2L tan[arc sin(hc/2dE)]

    Δ=2L tan[arc sin(hc/2dekVp)]

    arc tan(Δ/2L)=arc sin(hc/2dekVp)

where h is Planck's constant, c is the speed of light, E is the endpoint energy, and the other terms have been previously identified.Solving for the accelerating voltage, ##EQU1##

If only one-half of the mirror image shown in FIG. 2 is generated, i.e.,only a single spectrum, then energy as a function of position in thespectrum can be determined using the known energies of thecharacteristic lines of the x-ray tube target and the dispersionrelationship previously described. Because the characteristic linewavelengths and energies are well known, the energy of a spectralfeature at a fixed location in the image, such as the end point energy,can be easily determined from these known energies. The end point energyso determined is readily converted to an accelerating voltage since theend point energy equals e(kVp).

FIG. 3 is an enlargement of one of the "corners" of FIG. 2, i.e., wherethe continuous spectrum ends at the end point energy and the backgroundenergy begins. It is apparent from FIG. 3 that the end point energy canbe accurately determined based upon a linear fit of straight lines tothe measured image density data. From that figure, it is apparent thatthe peak accelerating voltage kVp applied to the x-ray tube to producethe image can be determined, according to the invention, from theintersection of two straight lines to an accuracy within 0.1 kV, asignificant improvement over the best techniques employed in the priorart.

The spectrograph and the images it produces can be calibrated, ifdesired, using the characteristic x-ray lines of known energy. Thedistances between those lines in the mirror image spectra can besimilarly used to establish a direct calibration. Likewise, a singlespectrum can be calibrated with the characteristic lines of knownenergy. A single calibration point is sufficient to define energies overthe entire spectrum using the known dispersion relationship and is,therefore, sufficient to determine a wide variety of acceleratingvoltages. Alternatively, accelerating voltages can be invasivelymeasured using a voltage divider and the traditional method of voltagecalibration to calibrate spectrographic images.

When the spectral image is formed on a photographic film, the film isanalyzed with a densitometer to locate the end point energy or energiesand to determine the accelerating voltage. Since the production ofphotographic images and their analysis requires considerable time, it ispreferable to provide an apparatus for automatic determination ofaccelerating voltages as shown in the schematic block diagram of FIG. 4.In that apparatus, the photographic film 22 is replaced by ascintillation screen 23, for example, of gadolinium oxysulfide. Thescintillation screen 23 produces light in response to incident x-raysand the scintillations are detected by a CCD camera 24 disposed directlyopposite the scintillation screen. In fact, it is preferred that thescintillation screen 23 be applied directly to the CCD camera 24. Inthis arrangement, the x-ray spectrum appears as a light image that isdetected by the CCD camera 24. In response, an electrical signal isproduced by the CCD camera. The electrical signal carries the image ofthe x-ray spectral distribution. The electrical signal is delivered to amicrocomputer 25 that automatically analyzes -the electrical signal. Asin the densitometer measurement of a photographic film, the analysisseeks the end point energy or energies where the continuous component ofthe x-ray radiation spectrum stops changing with energy i.e.,wavelength, and becomes constant, indicating a background energy levelnot related to x-ray emission from the source. That end point energyinformation is used in combination with previously supplied energycalibrations to determine the accelerating voltage applied to the x-raytube. The analysis is driven and completed by software stored in andused by the microcomputer 25. The microcomputer 26 may employ presentlyavailable spectral analysis software, such as the IMAGE programdeveloped by the National Institutes of Health. The results of thatdetermination are provided on a display 26 for use by the techniciancalibrating an x-ray apparatus.

The procedure employed to calibrate an x-ray apparatus acceleratingvoltage indicator is straightforward. Spectra are generated at variousvoltage settings of the voltage indicator 1 of the apparatus and the endpoint energies, i.e., the actual peak accelerating voltages applied tothe x-ray source, are determined from the spectra. The actual peakaccelerating voltages are determined either from photographic images orfrom electrical images using the apparatus illustrated in FIGS. 1 or 4.The apparatus of FIG. 4 permits a real time, rapid calibration of theaccelerating voltage indicator. A chart or graph of the actual peakaccelerating voltages applied versus the peak accelerating voltagesindicated by the apparatus is produced. The chart or graph is thenconsulted by a technician using the equipment for medical purposes toensure that the appropriate peak accelerating voltage is applied to thex-ray source to obtain proper contrast in an x-ray image produced for aparticular purpose.

The invention has been described with respect to certain preferredembodiments. Various additions and modifications within the spirit ofthe invention will occur to those of skill in the art. Accordingly, thescope of the invention is limited solely by the following claims.

I claim:
 1. A method of measuring an accelerating voltage applied to anx-ray source to produce x-rays comprising:applying an acceleratingvoltage to an x-ray source to produce x-ray radiation; diffractingportions of the x-ray radiation that are symmetrically disposed relativeto an axis with a single crystal material to produce two spectra of thex-ray radiation, each spectrum including continuous x-ray radiationhaving an end point energy at the maximum energy of the x-ray radiation;forming an image of the spectra of the x-ray radiation includingrespective end point energies; and measuring the separation of therespective end point energies of the spectra of the image and, thereby,determining the accelerating voltage applied to the x-ray source.
 2. Themethod of claim 1 including forming the image on a photographic film andlocating the end point energies by measuring the density of the image onthe film to locate the end point energies where the density of the imagestops changing with position and remains relatively constant.
 3. Themethod of claim 1 including forming the image on a scintillation filmand sensing the image with a charge coupled device camera on which thescintillation film is disposed.
 4. The method of claim 3 includinganalyzing the image formed on the charge coupled device camera andlocating the end point energies where the intensity of the image stopschanging with position and remains relatively constant.
 5. A method ofcalibrating an accelerating voltage indicator of an x-ray apparatuscomprising:applying an accelerating voltage to an x-ray source toproduce x-ray radiation and recording the accelerating voltage indicatedby an indicator of the x-ray apparatus; diffracting portions of thex-ray radiation that are symmetrically disposed relative to an axis witha single crystal material to produce two spectra of the x-ray radiation,each spectrum including continuous x-ray radiation having an end pointenergy at the maximum energy of the x-ray radiation; forming an image ofthe spectra of the x-ray radiation including respective end pointenergies; measuring the separation between the respective end pointenergies of the spectra and, thereby, determining the acceleratingvoltage applied to the x-ray source; comparing the accelerating voltageactually applied to the x-ray source to the indicated acceleratingvoltage; and repeating the foregoing steps for a plurality of differentaccelerating voltages to determine, for each applied acceleratingvoltage, the accelerating voltage actually applied and the indicatedaccelerating voltage, thereby calibrating the accelerating voltageindicator.
 6. A method of measuring an accelerating voltage applied toan x-ray source to produce x-rays comprising:applying an acceleratingvoltage to an x-ray source to produce x-ray radiation; diffracting aportion of the x-ray radiation with a single crystal material to producea spectrum of the x-ray radiation including continuous x-ray radiationhaving an end point energy at the maximum energy of the x-ray radiation;forming an image of the spectrum of the x-rays including the end pointenergy; and determining one of the wavelength and frequency of the endpoint energy and, thereby, the accelerating voltage applied to the x-raysource.
 7. The method of claim 6 including forming the image on aphotographic film and locating the end point energy by measuring thedensity of the image on the film to locate the end point energy wherethe density of the image stops changing with position and remainsconstant.
 8. The method of claim 6 including forming the image on ascintillation screen and sensing the image with a charge coupled devicecamera on which the scintillation screen is disposed.
 9. The method ofclaim 8 including analyzing the image formed on the charge coupleddevice camera and locating the end point energy where the intensity ofthe image stops changing with position and remains constant.
 10. Amethod of calibrating an accelerating voltage indicator of an x-rayapparatus comprising:applying an accelerating voltage to an x-ray sourceto produce x-ray radiation and recording the accelerating voltageindicated by an indicator of the x-ray apparatus; diffracting a portionof the x-ray radiation with a single crystal material to produce aspectrum of the x-ray radiation including continuous x-ray radiationhaving an end point energy at the maximum energy of the x-ray radiation;forming an image of the spectrum of the x-rays including the end pointenergy; determining one of the wavelength and frequency of the end pointenergy and, thereby, the accelerating voltage actually applied to thex-ray source; comparing the accelerating voltage actually applied to thex-ray source to the indicated accelerating voltage; and repeating theforegoing steps for a plurality of different accelerating voltages todetermine, for each applied accelerating voltage, the acceleratingvoltage actually applied and the indicated accelerating voltage, therebycalibrating the accelerating voltage indicator.
 11. An apparatus formeasuring an accelerating voltage applied to an x-ray source to producex-rays comprising:an x-ray spectrograph comprising an entrance foradmitting two portions of x-ray radiation emitted by an x-ray source; asingle crystal material disposed to intersect and disperse the portionsof the x-ray radiation to produce two x-ray spectra including continuousx-ray radiation having respective end point energies as the maximumenergy of the x-ray radiation; a scintillation screen emitting light inresponse to incident x-ray radiation for forming an image in light ofthe portions of the x-ray radiation dispersed by the single crystalmaterial; a charge coupled device camera disposed adjacent thescintillation screen for converting the image in light to electricalsignals; and a computer electrically connected to the charge coupleddevice camera for analyzing the electrical signals and finding theseparation between the end point energies in the image where theintensity of the x-ray spectrum stops changing and remains constant,thereby determining the accelerating voltage applied to the x-raysource.
 12. An apparatus for measuring an accelerating voltage appliedto an x-ray source to produce x-rays comprising:an x-ray spectrographcomprising an entrance for admitting a portion of x-ray radiationemitted by an x-ray source; a single crystal material disposed tointersect and disperse the portion of the x-ray radiation to produce anx-ray spectrum including continuous x-ray radiation having an end pointenergy as the maximum energy of the x-ray radiation; a scintillationscreen emitting light in response to incident x-rays for forming animage in light of the portion of the x-ray radiation dispersed by thesingle crystal material; a charge coupled device camera disposedadjacent the scintillation screen for converting the image in light toelectrical signals; and a computer electrically connected to the chargecoupled device camera for analyzing the electrical signals and findingthe end point energy in the image where the intensity of the x-rayspectrum stops changing and remains constant, thereby determining theaccelerating voltage applied to the x-ray source.